Depending on your backup format and destination, the minimum storage requirements to complete a backup may change.
Backups to remote destinations have different storage requirements. Archive backups with a remote destination create the archive locally and then upload it via SFTP to the remote host. Therefore, you must have free space equal to the size of the files being backed up (unless compression is enabled, in which case slightly less would be required). In a Direct copy backup with a remote destination, files are uploaded directly to the remote machine and therefore it is possible to backup a full drive. However, be warned that a remote direct copy is significantly slower than a archive backup.